1. Field of the Invention
The present invention relates to a connection of high frequency transmission lines, and more specifically, to the implementation of a connection portion between transmission lines which is capable of keeping a frequency characteristic in a good condition and of which a satisfactory connection strength can be maintained by easy means, through the use of substrates.
2. Description of the Related Art
Hitherto, the high frequency transmission lines has been connected by connecting, using a golden (Au) wire or an Au ribbon, two dielectric substrates on each of which a coplanar transmission line is formed on a signal line between grounds, or the high frequency transmission lines have been packaged and connected by a coaxial line via a connector.
However, under the recent tendency of transmission signals toward high frequencies, when the high frequency transmission lines are connected by an Au wire or an Au ribbon, an impedance-mismatched part in a connection portion can give rise of deterioration of high-frequency characteristic. This requires a reduction in the length of the Au wire or Au ribbon. Furthermore, with the tendency of transmission signals toward high frequencies, deterioration of high frequency characteristic due to displacements between transmission lines in the longitudinal direction and the height direction also becomes significant.
Therefore, considering positional displacements, position tolerance, and the like between the dielectric substrates, there is a limit to the connection between the substrates by an Au wire or Au ribbon. On the other hand, the way of connecting the substrates via the connector causes a higher cost, a larger size, the need for extra-length processing spaces of coaxial line and the like, and hence, this is unsuitable for size reduction or cost reduction. With such being the situation, in recent years, a method for connecting high-frequency transmission lines by using flexible substrates in connection portion has been prevalently employed.
That is because the use of impedance-matched flexible substrates allows the problem of the connection length and positional displacements between connection portions to be solved, thereby increasing the degree of freedom of the connection portion.
Techniques for connecting substrates utilizing flexible substrates are disclosed in Japanese laid-open Patents No.2005-101026, No.2005-26801 and No.2002-503033, and US published application No.US2004/0264882 A1.
When attempting to connect a transmission line on the flexible substrate and another transmission line to secure a band on the level of 40 GHz, it is difficult to keep a high-frequency characteristic in a good condition and maintain a satisfactory connection strength, at the connection portion between the transmission lines.
For an optical module, however, it is necessary that the connection portion between the substrate, and a laser device and/or a light-receiving device secures a band on the level of 40 GHz, keeping high-frequency characteristic in a good condition, and maintaining a satisfactory connection strength.
When attempting to secure a band on the level of 40 GHz and keep a high-frequency characteristic in a good condition, a problem of impedance mismatch can arise.
This problem is attributable to positional displacements and/or position tolerance between substrates, caused by the structure and material of the transmission lines at the connection portion between a substrate and a flexible substrate used for the connection between the substrate and a light receiving/emitting element.
When the surfaces of signal lines in coplanar lines on two substrates are directed in the same direction, the inter-ground distance of each of the two substrates is treated as being equivalent to an inductor, and hence, positional displacements and/or position tolerance between the substrates causes an impedance-mismatch, thereby resulting in degradation of high frequency characteristic.
With the above-described arrangement used, there is a high probability that stress will concentrate on the connection portion between the substrates to thereby break down a flying lead portion for connection.
On the other hand, when using a connection method in which the surfaces of signal lines in the coplanar lines on the flexible substrate and the dielectric substrate are connected face to face, the difference in the inter-ground distance between the two substrates can be reduced, but the problem associated with the strength of the connection portion remains to be solved.